1. Field of the Invention
This invention relates in general to a pressure head for bonding parts with heat and pressure, the head having a gimbaled horn that pivots on a push rod, wherein the shape and orientation of the push rod bearing surface corresponds to the shape and orientation of the contact surface with the part being bonded. More specifically, the bearing surface is elliptical and varies between being circular and linear.
2. Description of the Related Art
A thrust head or head is commonly used to attach one item to another. The head must move relative to the workpiece. In some applications the workpiece is moved into contact with the head. In other applications, the head is moved by a thruster, such as a pneumatic cylinder. The horn, that is the working element of the head, may be heated for bonding workpieces together, that is for bonding one article to another with heat and pressure. For example, a horn may bond a flexible circuit to an electronic component or may bond a flip chip to the electronic package circuitry.
In many applications, but particularly when heat and pressure are to be applied in the bonding, it is imperative that the contact surface of the horn Lace, align perfectly with the contact surface of the workpiece. One method of horn alignment to solve this problem has been to gimbal the horn such that, upon encountering the workpiece, the horn rotates to alignment.
In many applications with workpieces requiring very close bonding tolerances, it imperative that the force exerted on the horn by the workpiece required to align the gimbaled horn with the workpiece be extremely small. This is necessitated because the gimbaled horn not only rotates as it is aligned but also moves sideways thereby producing shear forces on the workpiece. If the rotational force is more than de minimis, then the shear force created in the rotation will disrupt the workpiece. Consequently, the mass of the gimbaled head needs to be minimized and the lever arm from the point of alignment force to the gimbal point or axis need to be maximized. Even the forces required to overcome the static friction of bearings in many conventional gimbaled horns or to overcome the moment and resistance to rotation created by wires leading to the heating element and thermocouple on the gimbaled horn are too large for use in some high tolerance applications.
My co-pending application, Ser. No. 08/806088 filed Feb. 25, 1997 titled PRESSURE HEAD WITH PIVOT ROD GIMBAL WITH ZERO ADJUST, describes a gimbaled horn not fixed in the head in the up/down direction, such that, upon first encountering the workpiece the horn can move within the head such that much lower shear forces are generated on the workpiece.
However, even with the above design, the shape and size of the bearing determines the magnitude of the moment forces required to overcome the bearing friction. Basically, the larger the bearing surface and the further from the pivot point, the larger the moment needed to overcome bearing friction.
Also, the shape of the workpiece also determines the amount of alignment force. For example, if the workpiece is long and narrow, then only small forces are needed at the longer distance to align the horn while large forces are required at the narrow distance to align the horn.
Therefore it is desirable that a gimbaled horn be adapted so that minimal force from the workpiece aligns the horn face with the workpiece.